Liquid crystalline polyesters (LCP's) are unique polymers because they possess good to excellent tensile, flexural, and temperature resistance (thermal stability) properties which are very desirable for high performance applications such as in structural and electronic applications. U.S. Pat. No. 4,169,933 discloses a group of liquid crystalline polyesters which consist essentially of residues of terephthalic acid, 2,6-naphthalenedicarboxylic acid, hydroquinone, and p-hydroxybenzoic acid. Although these known LCP's possess very good color, tensile, flexural, and thermal resistance properties, they are relatively expensive due, primarily, to the cost of the 2,6-naphthalenedicarboxylic acid and p-hydroxybenzoic acid monomers (or functional equivalents thereof) used in their preparation.
It, therefore, would be desirable to replace some of the monomers used in the manufacture of liquid crystalline polyesters with less expensive monomers, such as aliphatic or cycloaliphatic dicarboxylic acids, while maintaining the desirable properties of the LCP, e.g., good to excellent color, tensile, flexural, and thermal resistance properties. Those skilled in the art are aware that the use of such monomers as aliphatic or cycloaliphatic dicarboxylic acids in the manufacture of LCP's may result in an LCP which exhibits inferior properties, particularly inferior temperature resistance, flexural modulus, and overall level of liquid crystallinity, as compared to analogous, all-aromatic LCP's. Also, it is well known that highly liquid crystalline polymers exhibit shear-thinning behavior during injection molding, extrusion, and other processes in which the polymer is subjected to relatively high levels of shear, and any appreciable decrease in this shear-thinning behavior is undesirable. This characteristic is of particular importance during the injection of an LCP into very intricate molds in the injection molding of circuit boards having very thin sections that are widely used in the present age of computers and miniaturization. Further, it is well known that the use of more monomers in the preparation of a polymer usually reduces the melting point (&lt;400.degree. C. preferred), but the level of spherulitic crystallinity of the polymeric product also is very often reduced to an undesirably low level. A low level of spherulitic crystallinity may affect deleteriously the temperature resistance properties of the polymer.
It is an object of this invention to provide novel liquid crystalline polyester compositions. Another object of the invention is to provide liquid crystalline polyesters consisting essentially of certain levels of residues of terephthalic acid, cyclohexanedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, hydroquinone, 4,4'-biphenol and/or p-hydroxybenzoic acid that exhibit a combination of good mechanical and thermal resistance properties, and can be produced at a cost lower than that of the analogous all-aromatic LCP.